// This file is part of Eigen, a lightweight C++ template library
// for linear algebra.
//
// Copyright (C) 2016 Dmitry Vyukov <dvyukov@google.com>
//
// This Source Code Form is subject to the terms of the Mozilla
// Public License v. 2.0. If a copy of the MPL was not distributed
// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.

#ifndef EIGEN_CXX11_THREADPOOL_RUNQUEUE_H_
#define EIGEN_CXX11_THREADPOOL_RUNQUEUE_H_

namespace Eigen {

// RunQueue is a fixed-size, partially non-blocking deque or Work items.
// Operations on front of the queue must be done by a single thread (owner),
// operations on back of the queue can be done by multiple threads concurrently.
//
// Algorithm outline:
// All remote threads operating on the queue back are serialized by a mutex.
// This ensures that at most two threads access state: owner and one remote
// thread (Size aside). The algorithm ensures that the occupied region of the
// underlying array is logically continuous (can wraparound, but no stray
// occupied elements). Owner operates on one end of this region, remote thread
// operates on the other end. Synchronization between these threads
// (potential consumption of the last element and take up of the last empty
// element) happens by means of state variable in each element. States are:
// empty, busy (in process of insertion of removal) and ready. Threads claim
// elements (empty->busy and ready->busy transitions) by means of a CAS
// operation. The finishing transition (busy->empty and busy->ready) are done
// with plain store as the element is exclusively owned by the current thread.
//
// Note: we could permit only pointers as elements, then we would not need
// separate state variable as null/non-null pointer value would serve as state,
// but that would require malloc/free per operation for large, complex values
// (and this is designed to store std::function<()>).
template <typename Work, unsigned kSize> class RunQueue
{
public:
    RunQueue() : front_(0), back_(0)
    {
        // require power-of-two for fast masking
        eigen_plain_assert((kSize & (kSize - 1)) == 0);
        eigen_plain_assert(kSize > 2);            // why would you do this?
        eigen_plain_assert(kSize <= (64 << 10));  // leave enough space for counter
        for (unsigned i = 0; i < kSize; i++) array_[i].state.store(kEmpty, std::memory_order_relaxed);
    }

    ~RunQueue() { eigen_plain_assert(Size() == 0); }

    // PushFront inserts w at the beginning of the queue.
    // If queue is full returns w, otherwise returns default-constructed Work.
    Work PushFront(Work w)
    {
        unsigned front = front_.load(std::memory_order_relaxed);
        Elem* e = &array_[front & kMask];
        uint8_t s = e->state.load(std::memory_order_relaxed);
        if (s != kEmpty || !e->state.compare_exchange_strong(s, kBusy, std::memory_order_acquire))
            return w;
        front_.store(front + 1 + (kSize << 1), std::memory_order_relaxed);
        e->w = std::move(w);
        e->state.store(kReady, std::memory_order_release);
        return Work();
    }

    // PopFront removes and returns the first element in the queue.
    // If the queue was empty returns default-constructed Work.
    Work PopFront()
    {
        unsigned front = front_.load(std::memory_order_relaxed);
        Elem* e = &array_[(front - 1) & kMask];
        uint8_t s = e->state.load(std::memory_order_relaxed);
        if (s != kReady || !e->state.compare_exchange_strong(s, kBusy, std::memory_order_acquire))
            return Work();
        Work w = std::move(e->w);
        e->state.store(kEmpty, std::memory_order_release);
        front = ((front - 1) & kMask2) | (front & ~kMask2);
        front_.store(front, std::memory_order_relaxed);
        return w;
    }

    // PushBack adds w at the end of the queue.
    // If queue is full returns w, otherwise returns default-constructed Work.
    Work PushBack(Work w)
    {
        std::unique_lock<std::mutex> lock(mutex_);
        unsigned back = back_.load(std::memory_order_relaxed);
        Elem* e = &array_[(back - 1) & kMask];
        uint8_t s = e->state.load(std::memory_order_relaxed);
        if (s != kEmpty || !e->state.compare_exchange_strong(s, kBusy, std::memory_order_acquire))
            return w;
        back = ((back - 1) & kMask2) | (back & ~kMask2);
        back_.store(back, std::memory_order_relaxed);
        e->w = std::move(w);
        e->state.store(kReady, std::memory_order_release);
        return Work();
    }

    // PopBack removes and returns the last elements in the queue.
    Work PopBack()
    {
        if (Empty())
            return Work();
        std::unique_lock<std::mutex> lock(mutex_);
        unsigned back = back_.load(std::memory_order_relaxed);
        Elem* e = &array_[back & kMask];
        uint8_t s = e->state.load(std::memory_order_relaxed);
        if (s != kReady || !e->state.compare_exchange_strong(s, kBusy, std::memory_order_acquire))
            return Work();
        Work w = std::move(e->w);
        e->state.store(kEmpty, std::memory_order_release);
        back_.store(back + 1 + (kSize << 1), std::memory_order_relaxed);
        return w;
    }

    // PopBackHalf removes and returns half last elements in the queue.
    // Returns number of elements removed.
    unsigned PopBackHalf(std::vector<Work>* result)
    {
        if (Empty())
            return 0;
        std::unique_lock<std::mutex> lock(mutex_);
        unsigned back = back_.load(std::memory_order_relaxed);
        unsigned size = Size();
        unsigned mid = back;
        if (size > 1)
            mid = back + (size - 1) / 2;
        unsigned n = 0;
        unsigned start = 0;
        for (; static_cast<int>(mid - back) >= 0; mid--)
        {
            Elem* e = &array_[mid & kMask];
            uint8_t s = e->state.load(std::memory_order_relaxed);
            if (n == 0)
            {
                if (s != kReady || !e->state.compare_exchange_strong(s, kBusy, std::memory_order_acquire))
                    continue;
                start = mid;
            }
            else
            {
                // Note: no need to store temporal kBusy, we exclusively own these
                // elements.
                eigen_plain_assert(s == kReady);
            }
            result->push_back(std::move(e->w));
            e->state.store(kEmpty, std::memory_order_release);
            n++;
        }
        if (n != 0)
            back_.store(start + 1 + (kSize << 1), std::memory_order_relaxed);
        return n;
    }

    // Size returns current queue size.
    // Can be called by any thread at any time.
    unsigned Size() const { return SizeOrNotEmpty<true>(); }

    // Empty tests whether container is empty.
    // Can be called by any thread at any time.
    bool Empty() const { return SizeOrNotEmpty<false>() == 0; }

    // Delete all the elements from the queue.
    void Flush()
    {
        while (!Empty()) { PopFront(); }
    }

private:
    static const unsigned kMask = kSize - 1;
    static const unsigned kMask2 = (kSize << 1) - 1;
    struct Elem
    {
        std::atomic<uint8_t> state;
        Work w;
    };
    enum
    {
        kEmpty,
        kBusy,
        kReady,
    };
    std::mutex mutex_;
    // Low log(kSize) + 1 bits in front_ and back_ contain rolling index of
    // front/back, respectively. The remaining bits contain modification counters
    // that are incremented on Push operations. This allows us to (1) distinguish
    // between empty and full conditions (if we would use log(kSize) bits for
    // position, these conditions would be indistinguishable); (2) obtain
    // consistent snapshot of front_/back_ for Size operation using the
    // modification counters.
    std::atomic<unsigned> front_;
    std::atomic<unsigned> back_;
    Elem array_[kSize];

    // SizeOrNotEmpty returns current queue size; if NeedSizeEstimate is false,
    // only whether the size is 0 is guaranteed to be correct.
    // Can be called by any thread at any time.
    template <bool NeedSizeEstimate> unsigned SizeOrNotEmpty() const
    {
        // Emptiness plays critical role in thread pool blocking. So we go to great
        // effort to not produce false positives (claim non-empty queue as empty).
        unsigned front = front_.load(std::memory_order_acquire);
        for (;;)
        {
            // Capture a consistent snapshot of front/tail.
            unsigned back = back_.load(std::memory_order_acquire);
            unsigned front1 = front_.load(std::memory_order_relaxed);
            if (front != front1)
            {
                front = front1;
                std::atomic_thread_fence(std::memory_order_acquire);
                continue;
            }
            if (NeedSizeEstimate)
            {
                return CalculateSize(front, back);
            }
            else
            {
                // This value will be 0 if the queue is empty, and undefined otherwise.
                unsigned maybe_zero = ((front ^ back) & kMask2);
                // Queue size estimate must agree with maybe zero check on the queue
                // empty/non-empty state.
                eigen_assert((CalculateSize(front, back) == 0) == (maybe_zero == 0));
                return maybe_zero;
            }
        }
    }

    EIGEN_ALWAYS_INLINE
    unsigned CalculateSize(unsigned front, unsigned back) const
    {
        int size = (front & kMask2) - (back & kMask2);
        // Fix overflow.
        if (size < 0)
            size += 2 * kSize;
        // Order of modification in push/pop is crafted to make the queue look
        // larger than it is during concurrent modifications. E.g. push can
        // increment size before the corresponding pop has decremented it.
        // So the computed size can be up to kSize + 1, fix it.
        if (size > static_cast<int>(kSize))
            size = kSize;
        return static_cast<unsigned>(size);
    }

    RunQueue(const RunQueue&) = delete;
    void operator=(const RunQueue&) = delete;
};

}  // namespace Eigen

#endif  // EIGEN_CXX11_THREADPOOL_RUNQUEUE_H_
